Project summary: Streptococcus mutans is the primary cariogen that produces several virulence factors. The long-term goal of the proposed studies is to characterize specific interactions of S. mutans with oral bacteria in the dental plaque biofilm. We hypothesize that it is not merely colonization by S. mutans that determines the ultimate cariogenicity of plaque, but the interaction of the pathogen with other biofilm bacteria that affect virulence. Other bacteria present in dental plaque may modulate quorum sensing-dependent expression of virulence properties of the organism. This hypothesis is based on observations that: 1) S. mutans competence-stimulating peptide (CSP) and a signal transduction system mediate a variety of virulence characteristics: biofilm formation, bacteriocin production, acid tolerance, antimicrobial sensitivity, and natural genetic transformation. Elimination of CSP or its signal transduction system results in alterations in all of these virulence activities. 2), other oral bacteria, such as the commensal early-colonizing dental plaque species Streptococcus gordonii, can inactivate S. mutans CSP thereby interfering with S. mutans bacteriocin production. Therefore, the experimental focus of this proposal will be to determine the effects of other early colonizers such as Actinomyces naeslundii and later anaerobic colonizers such as Porphyromonas gingivalis on quorum-sensing in S. mutans and to determine the effects of these organisms on the virulence-associated properties of S. mutans. The Specific Aims are: Specific Aim 1. To determine if early colonizers, S. gordonii or A. naeslundii, antagonize colonization efficiency and the acid tolerance response in S. mutans. i) Colonization of S. mutans WT and its comC mutant on preformed biofilms of S. gordonii Challis or A. naeslundii will be assessed by both radioactive labeling techniques and confocal microscopy. ii) The acid tolerance response of S. mutans and mixtures with S. gordonii Challis or A. naeslundii will be assessed by viable cell counts. Specific Aim 2. To determine if late colonizers such as P. gingivalis, Treponema denticola, and Tannerella forsythia antagonize quorum-sensing properties in S. mutans. i) S. mutans bacteriocin production in broth monocultures of S. mutans and mixtures of S. mutans with P. gingivalis, T. denticola, or T. forsythia will be monitored by determining killing percentages of an indicator strain. ii) The transformation efficiencies of S. mutans in monocultures and in mixtures with P. gingivalis, T. denticola, or T. forsythia will be assessed. iii) The acid tolerance response of S. mutans and mixtures with P. gingivalis, T. denticola, or T. forsythia will be assessed by viable cell counts. iv) We will determine if the proteinases of P. gingivalis inactivate the S. mutans CSP. Project Narrative: Dental caries is a common chronic infectious disease affecting much of the worldwide population. The S. mutans quorum-sensing system may be an appropriate target for interruption to reduce dental caries. The disruption of CSP-dependent regulation has the potential to attenuate the bacterium so that it is less virulent.